Fluid meters, for example, those used to measure a flow of natural gas petroleum products or water, often have a shaft with one or more blades or vanes that are rotated by a flow of fluid through the meter. A magnet is often mounted on the shaft and produces a rotating magnetic field that, by mechanical, electrical, or electronic means, conveys flow information to a totalizing or rate measuring apparatus. The rotating field is often used to cause rotation of a shaft-mounted secondary magnet that, in turn, drives a mechanical totalizer or a magnetically sensitive detector generating pulses in an electrical circuit.
Typically, in order to be close to the rotating magnet, the magnetically sensitive detector is located in a cup approximately one-half inch in diameter, which is set into a side of a meter housing. Inside the meter, the cup is surrounded by the rotating magnet; and hence, the magnetically sensitive detector is able to a rotating magnetic field from the magnet. While a magnetically sensitive detector installed this way provides accurate fluid flow measurements, it does have some disadvantages. First, the magnetically sensitive detector is often mounted on a printed circuit board assembly that contains electrical components necessary to interface with the detector. Further, the printed circuit board assembly must be sized to fit in the cup in the meter housing; and that requirement places significant design limitations on the detector circuitry, which often leads to an increase in cost.
A second disadvantage is that different size fluid meters often have different size cups into which the magnetically sensitive detector is to be located; and the requirement to design, manufacture and inventory different sizes of detectors to fit the various sizes of cups is also costly. A third disadvantage relates to the construction of many meter housings. The hole in which the magnetically sensitive detector is located is capped by a metal plate inside the housing that is sealed with a gasket to prevent fluid leaks. Over time, it is possible for the gasket to fail; and the fluid then leaks into the hole, thereby exposing the detector to corrosive effects, if any, of the fluid.
Therefore, there is a need to provide a capability of sensing the rotation of a magnet within the meter housing, which does not have the disadvantages discussed above.